500 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



by the aid of emulsin. In certain cases poisonous substances arise from 

 the co-operation of two distinct organisms, as, for example, in conjoint 

 infection (cf. Fliigge, 1. c., p. 309). 



SECTION 90. Ethereal Oils, Resin, &c. 



Ethereal oils, balsams, resins, india-rubber, &c., are as a general rule 

 not reassimilated, and even in an extremely fine state of division or in the 

 form of an emulsion they do not afford a suitable medium for the growth 

 of fungi. Ethereal oils serve mainly for purposes of attraction, and also 

 affect the diathermanous character of the surrounding air, whereas balsams 

 and latex serve to heal wounds, or by impregnating the cell-wall influence 

 its permeability 1 . 



These substances may be formed within the protoplast, and may 

 appear either as an emulsion (latex, &c.) or may coalesce to form 

 homogenous masses of oil or balsam, the products being either permanently 

 retained at the point of origin or excreted, internally or externally. 

 Substances may apparently also be excreted which become converted 

 outside the protoplast into ethereal oils or balsams, and the latter may 

 even arise by metamorphosis of the cell-wall. The details of these pro- 

 cesses are, however, quite unknown, in spite of the careful histological 

 study to which the special glands, gland-hairs, secretory receptacles' 2 , &c. 

 have been subjected. Many resins and forms of latex undergo certain 

 changes when exposed to air, and it is by means of a post-mortem 

 absorption of oxygen that the ethereal oil of the camphor-tree is converted 

 into camphor. 



Both the production and excretion of the substances mentioned may 

 be influenced by external circumstances, as when a pathological formation 

 of resin occurs, or when a plant becomes more odoriferous in a warm and 

 sunny habitat. Insolation usually causes ethereal oils to evaporate more 

 rapidly, and in Dictamnus albus the air around the inflorescence may 

 actually be capable of ignition. Many flowers are most fragrant at night, 

 either because the production of the ethereal oils is most active at that 

 time, or because spme special causes induce more rapid exhalation 3 . 



1 Cf. Stahl, Ludwig, Kemer, &c. The marked absorption of dark heat-rays by the vapours of 

 ethereal oils can hardly be of any great importance as a protection against insolation. 



a Cf. de Bary, Anatomic, 1877, pp. 72, 152 ; Tschirch, Pflanzenanatomie, 1889, p. 460; Haber- 

 landt, Physiol. Pflanzenanat., 1896, 2. Aufl., p. 432 ; Tschirch, Jahrb. f. wiss. Bot., 1893, Bd. xxv, 

 p. 370; Siek, ibid., 1895, Bd. xxvil, p. 238; Becheraz, Bot. Centralbl., 1894, Bd. LX, p. 20; 

 Berthold, Protoplasmamechanik 1886, pp. 14, 27. 



3 Treviranus, Physiol., Bd. I p. 93 ; de Candolle, Physiol., T. II, p. 764 ; Regel, Bot. Centralbl., 

 1891, Bd. XLV, p. 343; Mesnard, Ann. d. sci. nat., 1894, vii. se*r., T. xvin, p. 374; Rev. gen. d. 

 Bot, 1894, T. vi, p. 97, and 1896, T. vin, p. 129. 



